What evidence is there for the earth being billions of years old?

The scientific evidence is strong, however. It was known by geologists that the earth is “ancient” even before the 19th century. In the early 19th century, names were given to the strata, e.g. Devonian, named after the British county of Devon where the Devonian stratum was first described. This is now known to be about 400 million years old, but at that time the layers could only be dated relative to each other, not absolutely.

In the 20th century techniques were developed to measure the ages of rocks by using the decay rates in radiometric rocks from a “mother” to a “daughter” isotope, allowing absolute dates to be determined. One of these techniques uses an isochon diagram:

For rocks for which this is appropriate, the method shows, using different minerals in the rock sample, three things:

(a) A straight line confirms that there has not been intrusion of either mother or daughter isotope, and the isochron is “simple”, meaning that during its history there were no interruptions due to major geological events.

(b) The initial amount of the daughter isotope is indicated by where the line intersects the y-axis.

(c) The rate of decay is known, so the slope of the line shows the age of the rock sample.

This is not done by hand-waving over a graph. Statistical techniques are used to infer the significance level.

It is sometimes objected (although never by experts) that radiometric dating is unsound because:

(a) The initial of the daughter amount of the isotope is not known. Sometimes this can be true, so great caution is exercised by the scientist. However in the case of a simple isochron, the initial daughter amount is actually computed.

(b) There might have been corruption of either isotope in the geological history of the rock. That can happen, and scientists are very careful to exclude that possibility, by confirming that the line on the graph is straight.

(c) The rate of decay may have been very much greater in the past. There is not the slightest scientific evidence that this is the case. However if it is supposed that the rate of decay was sufficiently higher in the past than the present-day to explain isochron graphs with a young age of the rocks, then the amount of energy released would have been enough to evaporate the surface of the earth.

A number of different isotope pairs have been used, and have produced very similar results. The earth is just over 4.5 billion years old.